There is a continuing expansion of the use of composite materials for a diverse array of applications, especially in the aerospace field. One challenge confronting manufacturing engineers is to enhance the reproducibility of composite articles having complex configurations, e.g., to reduce the complexity of molding assemblies required to fabricate the composite article, to reduce the overall time of the manufacturing cycle required to fabricate the composite article, and to reduce the degree of manual labor required in the manufacturing cycle. This challenge is particularly pertinent in the fabrication of composite articles having a relatively complex box-beam cross-sectional configuration (complexity as used in the present context defines a composite article having a plurality of mechanical joints, i.e., locations where two or more box-beam members are structurally integrated in combination).
Of particular interest to the assignee of the present invention is enhancing the reproducibility of the structural framework of the canopy assembly of the RAH-66 Comanche helicopter (see FIGS. 1A-1C) under development by the assignee. The structural framework of this canopy assembly has a relatively complex box-beam cross-sectional configuration. Several conventional manufacturing techniques were evaluated by the manufacturing engineers of the assignee in its quest to enhance the reproducibility of the structural framework in an optimal manner, e.g., low cost, low manufacturing cycle time, low risk.
One conventional technique that was evaluated was the fabrication of the inner and outer segments separately and then subsequently joining the inner and outer segments to form the structural framework. Generally, this technique involved a relatively complex molding assembly comprising a plurality of complementary molds/tools that define the inner and outer mold line surfaces of the inner and outer segments. Due to the relatively complex configuration of the structural flamework, the optimal technique required, at a minimum, seven complementary molds/tools to fabricate the inner and outer segments (the inner segment would be fabricated as six separate components, the outer segment would be fabricated as a single component) comprising the structural framework. As one skilled in the art will appreciate, the initial capitalization costs required to provide the requisite number of sets of complementary molds/tools for full-scale production of the structural framework would greatly increase the per unit costs associated with the structural framework. In addition, the overall manufacturing cycle time and the man-hours required would be relatively high due to the necessity of fabricating seven different components for each structural framework.
Once the inner and outer segments have been fabricated as individual components, a technique must be selected for joining the individual components to form the structural framework. One conventional joining technique involves the use of mechanical fasteners to join the inner and outer segments in combination. While the use of mechanical fasteners is a satisfactory technique, there are several drawbacks involved in the use of mechanical fasteners. First, the use of mechanical fasteners is highly labor intensive, e.g., drilling the necessary fasteners holes, installing the fasteners, which significantly increases the overall per unit costs associated with the structural framework as well the overall manufacturing cycle time. For the structural framework described hereinbelow in further detail, it was estimated that eight hundred mechanical fasteners would be required to mechanically join the inner and outer segments. In addition, since the structural framework has a box-beam cross-sectional configuration, conventional upset-type mechanical fasteners cannot be used since the box-beam configuration does not permit access to the interior of the structural framework for the upsetting procedure. Hence, blind fasteners must be used, and blind fasteners are relatively costly (approximately $10/fastener). Further, the use of blind fasteners entails a high risk since the integrity of the mechanical connections provided by the blind fasteners cannot be visually verified due to the box-beam configuration.
Another conventional joining technique involves the use of a bonding agent, e.g., a thermoplastic material such as polyetheretherketone (PEEK), to join the inner and outer segments in combination. While this technique is also satisfactory, there are several drawbacks. A bonding fixture is required to maintain the various components comprising in the inner and outer segments in proper bonding position. Such bonding fixtures are expensive, due to the necessity to fabricate such fixtures to very precise dimensional tolerances, and represent an initial capital outlay for full-scale production that would increase the per units costs associated with the structural framework. In addition, it may be difficult to achieve a satisfactory bond between the inner and outer segments since it is difficult to exert an outward pressure between the inner and outer segments due to the box-beam configuration. Further, it is difficult to ensure that sufficient thermal energy is provided to the bonding agent to form an acceptable bond.
A need exists to provide a method for enhancing the reproducibility of composite articles having a box-beam cross-sectional configuration such as the structural framework of a helicopter canopy assembly. The method should eliminate the need to utilize a relatively complex molding assembly, i.e., a plurality of complementary molds/tools that define the inner and outer mold line surfaces of the inner and outer segments comprising the composite article to be fabricated, and/or an expensive bonding fixture.